Cleveland police routinely engage in “unreasonable and unnecessary” force, exemplified by a half-hour police chase involving 100 officers that left two unarmed African Americans dead when police mistook the car backfiring for gunshots and shot each of them more than 20 times, a Justice Department investigation revealed Thursday.

“The investigation concluded that there is reasonable cause to believe that Cleveland police engage in a pattern or practice of unreasonable force in violation of the 4th Amendment,” Atty. Gen. Eric H. Holder Jr. said Thursday. “Our investigation revealed that the causes of these patterns or practices were systemic and resulted from organizational deficiencies.”

The probe, part of an ongoing series of “pattern or practice” investigations into the nation’s police departments, also found that Cleveland police often needlessly shot residents, struck them with head blows and subjected them to Taser weapons and chemical spray.

Taken together, the incidents in Ohio’s second-largest city, the Justice Department concluded, have led to a situation where “avoidable force becomes inevitable.”

Faced with the federal probe's findings, Cleveland police and city officials have signed a statement of principles committing them to mending police-community relations. Holder said the plan will lead to a consent decree that would be “court-enforceable,” with an independent monitor to oversee improvements and ensure that reforms are made.

“There are problems with the force here that have been identified. I also think the people of Cleveland should have a sense of hope that these problems can be rectified. ... The relationships must be made better,” Holder said.

Similar agreements have been reached after Justice Department investigations into police departments in other communities in states including California, Arizona, New Mexico and Louisiana.

Holder, who announced the Cleveland findings a day after he opened a separate investigation into the chokehold death of an unarmed black man in New York, recommitted his office to the Obama administration’s Building Community Trust initiative.

The effort is designed to “foster strong, collaborative relationships between local police and communities they protect and serve,” the attorney general said.

In Cleveland, Holder said, the issues of police and community relationships are “complex and the problems longstanding.” But, he said, “we have seen in city after city where we have engaged that meaningful change is possible.”

The Cleveland probe was opened after a local newspaper, the Plain Dealer, revealed in May 2011 that six officers accused of brutality had used force on 29 suspects during a two-year period.

The paper also reported that police management often “overlooked” conflicting statements in use-of-force investigations, leading to a high clearance rate for abusive officers.

But the most egregious incident cited by the Justice Department occurred on Nov. 29, 2012, when more than 100 officers launched the high speed-chase that killed Timothy Russell and Malissa Williams. The pair had driven by the local Justice Center in downtown Cleveland when officers heard a noise they took to be gunfire.

The massive chase, involving at least 62 police vehicles, some of them unmarked, lasted 25 minutes at speeds as high as 100 miles an hour. When Russell and Williams were finally cornered outside the city limits, 13 officers fired 137 shots at the car, killing both of them.

In all, about 37% of the on-duty force was involved in the incident. It turned out the initial “gunshots” were actually from the car’s muffler backfiring.

“The incident inflamed community perceptions, particularly in the African American community,” and convinced the public that the Cleveland Police Department was “out of control and that its officers routinely engaged in brutality,” the Justice report said.

One officer who jumped on the hood of the car and fired 15 shots was indicted earlier this year by local authorities for voluntary manslaughter in connection with that incident. Awaiting trial, he has denied the charges.

At Cleveland police headquarters, the department “now admits that the manner in which the chase occurred was not in accordance with established policies,” the Justice report said.

On Wednesday, a grand jury in New York declined to indict a white police officer for applying a chokehold to Eric Garner on Staten Island, leading to the unarmed African American’s death. Last week, a grand jury in St. Louis County decided against indicting a white policeman for shooting to death Michael Brown, another unarmed African American.

“We announce these findings today against the backdrop of the events in Ferguson and New York and in the wake of a shooting here in Cleveland of a 12-year-old boy just days ago,” Holder said. “These events have spawned a national conversation about police practices, community-police trust, and public safety.”